>Hi Bob and all,
>
>You wrote:
>\
>> Late versions of the SB-220 and, I suppose all of the SB-221 runs, had a
>> different capacitof for C1 which had just a little bit wider plate
>> spacing in the design. This was an attempt to stop arcing and spitting
>> when the parasitics might occur.
>That's utter nonsense.
>
Does Mr. Rauch's 'nonsense' claim check out?
>The spitting and sputtering is not due to parasitics at all.
>
>The plate variable/ tank arcing problem most often is incorrect
>tuning, and secondarily incorrect relay sequencing in the relay
>contacts.
>
It is true that hotswitching provides current-transients which tend to
ring the anode's VHF-resonant circuit -- which sometimes results in VHF
parasites in marginally-stable amplifiers. However, by installing high
speed switching, arcing can be reduced, possibly leading the amplifier
operator to assume that the problem has been absolutely, positively
cured..
- During the grate parasitics debate, Mr. Rauch claimed that Tune-C
arcing in the SB-220 could be produced by incorrect tuning. I tried to
make my SB-220 arc by mistuning it. I observed no arcs. A chap who was
monitoring the debate said that he measured the maximum peak voltage
across the Tune C at 3600V during deliberate mistuning. This made sense
to me because the withstanding V capability of the Tune C in my SB-220
measured close to 3600v.
>What happen in ANY less than 360 degree plate current conduction PA
>is the tube "tugs" on the tank (remember the tube is a varying
>resistance, not a source).
good point
>The tank flywheels (or more correctly acts
>like a pendulum) and swings past zero volts negative from it's Q.
>
This statement does not make sense to me.
- The negative half of tank voltage swing occurs during anode conduction
-- which is not during the flywheel part of the cycle. The flywheel
effect occurs during the not-conducting positive half of the cycle.
>If you underload the PA, or remove the load through ANY type of
>problem ...snip...
Mr. Rauch has been quick to blame load faults for what appears to be a
VHF instability issue. . . Most of the tank arcing that I have heard
about occurs after the amplifier was tuned up, when no load fault took
place.
>
>Normal fully loaded PEAK voltage in the tank of an AB class amplifier
>is about equal to (only slightly less than) the supply voltage.... ...
... which in the SB220 is normally 2900v -300v = 2600v peak.
>(in the case of the tank it swings +
>and - because of the blocking cap).
>
>But if you underload the tank for ANY reason, the voltage can easily
>reach three times the dc voltage.
>
Three times the anode supply potential of 2900v would be 8700v. It makes
little sense to me that the anode voltage could possibly soar to such
heights when a tank is essentially out of resonance.
- According to measurements made during the VHF parasitics debate, the
peak potential across the SB-220 Tune-C during deliberate mistuning was
about 3600v {see KN6DV's parasitics debate archive}
>The same is true if your exciter has a power transient (common with
>many rigs), because overdrive is like underloading so far as the
>anode system is concerned.
In an SB-220, saturation begins to occur at roughly 150W drive.
Increasing the drive beyond 180w does essentially nothing to the output
level or to the peak voltage across the Tune C.
... ...
Rich---
R. L. Measures, 805-386-3734, AG6K
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